This invention relates to a valve timing retarding system and more particularly to an improved arrangement for adjusting the valve timing between the intake and exhaust valves of an internal combustion engine.
It is well known that the valve timing of an internal combustion engine is highly important in determining the engine performance. In order to achieve maximum power output, relatively large overlaps between the timing of the intake and exhaust valves are desirable. However, such large overlaps tend to deteriorate the performance at lower engine speed. Therefore, it has been proposed to employ a variable timing arrangement wherein the duration of overlap can be changed with minimum overlap being provided at low engine speeds and maximum overlap being provided at wide open throttle. A wide variety of mechanisms have been proposed for this purpose.
Frequently, engines employ twin overhead camshafts with one camshaft serving the function of opening all of intake valves with the other camshaft serving the function opening all of the exhaust valves. When such an arrangement is employed, it is desirable frequently to change the timing of both the intake and the exhaust camshafts relative to the engine output shaft. If a timing adjusting mechanism is employed between the two camshafts, then it is necessary to somehow incorporate this timing adjustment into the chain tensioner or drive mechanism between the two camshafts. Alternatively, it has been proposed to provide separate adjusters between the drive and each camshaft. However, when such separate timing adjusters are provided at the same end of the engine, the construction becomes quite large and complicated.
It is, therefore, a principal object of this invention to provide an improved camshaft drive for a twin camshaft engine wherein the timing relationship of each camshaft may be adjusted and yet a compact drive arrangement and adjusting mechanism is incorporated.
It is a further object of this invention to provide an improved camshaft driving arrangement and timing adjustment mechanism for a twin camshaft internal combustion engine.
It is yet a further object of this invention to provide separately adjustable timing mechanisms for the intake and exhaust camshafts of an engine with the adjusting mechanisms being located at different locations so as to permit a more compact assembly.
Many adjusting mechanisms for the camshaft timing employ hydraulic operators. However, when the camshaft timing is adjusted hydraulically, certain problems can arise. That is, frequently, the hydraulic adjusting mechanism must operate against the driving torque of the camshaft. This means that the hydraulic pressure necessary to accommodate the timing adjustment may vary with the engine load. In some instances, the arrangement can be such that the load itself can overcome the adjusting mechanism and effect an unwanted timing adjustment.
It is, therefore, a further object of this invention to provide an improved timing adjustment mechanism for a camshaft drive including a hydraulic mechanism and wherein adjustment is not permitted except when hydraulic actuation is being experienced.
It is a further object of this invention to provide an improved hydraulic timing adjuster for an internal combustion engine camshaft mechanism that will be self-locking when not being actuated.